Electro-optical transmission



A. G. COOLEY Nov. 29, 1938.

ELECTED-OPTICAL TRANSMISSION METHOD AND APPARATUS Filed Oct. 26, 1956 2Sheets-Sheet l 2 Sheets-Sheet 2 INVENTOR BY/ZQ/d/%,W

ATTORNEY Nov.29,1938. A. G. CO'QLEY ELECTRO-OPTICAL TRANSMISSION METHODAND APPARATUS Filed 06x. 26, 1936 u7/Ill/fill!!!I'WIIIIIIIIIIIIIAillllllllllmmllllllllllllif lllllll)! Patented Nov. 29, 1938 UNITEDSTATES- PATENT OFFICE ELECTED-OPTICAL TRANSMISSION METHOD AND APPARATUSAustin G. Cooley, Rutherford, N. 5., assignor to Wide World Photos,Inc., New York, N. Y., a corporation of New York Application October 26,1936, Serial No. 107,545

18 Claims.

, ing and synthesizing apparatus at the transmitter and receiver of anelectro-optical system e. g., a facsimile or photo-transmission systemand the like.

Another object is to provide an improved system of electro-opticaltransmission of the type wherein the picture current, or a selectedfrequency component thereof, is used for synchronizing; in conjunctionwith novel means to compensate for frequency drift introduced duringtransmission.

A feature ofthe invention relates to an improved form of compensatingdevice to compensate for frequency drift in a carrier currenttransmitting and receiving system.

Another feature relates to an improved organzation of apparatus formaintaining synchronism in a photo-telegraphic, facsimile or similarsystem. i

A further feature relates to the novel organization, arrangement andconnection of parts which constitute an improved synchronizingmonitoring system for systems employing a carrier wave link between atransmitting station and a receiving station.

Other features and advantages not specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

While the invention will be disclosed herein as embodied in one knowntype of signal transmission system, it will be understood that this isdone merely for explanatory purposes and not by way of limitation.Furthermore only those parts of the system are shown as are necessary toa complete understanding of the inventive concept. Accordingly in thedrawings,

Fig, l is a schematic diagram of an electrooptical transmission systemembodying features of the invention.

Fig. 2 is a detailed wiring diagram, partly schematic, of the systemoutlined in Fig. 1.

Fig. 3 is a characteristic representation of a typical wave-train usedat one stage of the synchronizing process according to the invention.

Fig. 4 is a detailed view, partly sectional, of the improved frequencydrift compensator according to the invention.

Referring for the present to Fig. l, the numeral i represents generallyany well-known form of apparatus for translating the shade variations ofa visual representation into so-called picture currents. Preferably,although not necessarily, 5 this apparatus is of the type disclosed inPatent No. 2,015,742, granted October 1, 1935, wherein the picturecurrents are produced in the form of a variable amplitude wave having aninherent frequency component or fundamental definitely related to thespeed of the movable analyzing mechanism. In general there are twomethods of transmitting the picture currents to a receiving station,namely by transmitting the currents directly over a metallic circuit, orby employing them to modulate a carrier wave which may be transmittedeither over a metallic circuit or by means of radio channels or thelike. When the picture signals are transmitted directly over a metallicline, such for example as the usual telephone service line, satisfactorysynchronization can be achieved. On .the other hand, I have found thatwhen these picture currents are transmitted over the usual carriertelephone channels, there may be introduced into the re- 2 producedpicture a noticeable distortion. This distortion is due to the fact thatthe frequency of the modulating and demodulating oscillators of thecarrier telephone channels are not always synchronized, resulting inwhat may be termed frequency drift. While this frequency drift is notobjectionable in ordinary telephonic or sound wave communication, it isat times very objec-. tionable in picture transmission. In accordancewith the present invention means are provided to compensate for thisfrequency drift so that its effect on the picture reproduction isreduced to a substantially negligible amount. Thus as indicated in Fig.1 of the drawings, the output of the picture transmitter i is applied tothe usual telephone. service line 2, and at the telephone central ofliceterminal equipment, the picture currents are transformed into acorrespondingly modulated carrier wave. Merely for purposes ofillustration this carrier equipment is represented schematically in Fig.l by the carrier oscillator 3 and the modulator i. The modulated carrierwave is then transmitted over a suitable carrier channel 6, either wireor wireless, to a carrier telephone receiving station at which islocated any well-known-form of demodulator. As is generally the casethese demodulators employ some form of local receiving oscillator l anddemodulating equipment by 1 means of which the original picture signalsare capable of being demodulated or detected and impressed. on the localtelephone link 9 leading to the picture receiving equipment 10. Forpurposes of ordinary sound wave communication it is not necessary tomaintain the frequency stability of the oscillator i above apredetermined amount with the result that from time to time there may beproduced a frequency drift of as much as ten cycles per second in thedemodulated wave. Under these circumstances, any system of picturetransmission which employs the original picture signals as a medium ofsynchronization will be subjected to variations resulting in lack ofsynchronism between the picture transmitter and the picture reproducer.To remedy this condition, the synchronous motor H which drives thepicture reproducing drum or carrier i2 is controlled by the signals fromthe .line 9, after they have been acted upon by suitable driftcompensating mechanism represented by the numeral i3 and shown in detailin Figs. 2' and 4.

In general the drift compensator comprises a I rotary transformer havinga two phase primary supplied by the incoming picture signal, and asingle phase secondary which feeds the circuits which supply thesynchronous motor ll. By varying the relative speed of rotation betweenthe primary and secondary windings, the fundamental frequency of thereceived picture signals may be increased or decreased in the properamount to render them synchronous with the signals as originallygenerated at the device i. In order that the equipment may be used onordinary non-carrier telephone transmission lines, a switch it isprovided to disconnect the drift compensator it. When the driftcompensator is in circuitit is necessary to provide some indication tothe attendant when the device it is in synchronism with thecorresponding device at the transmitter i. For this purpose thesynchronous motor l i may drive a stroboscopic chart and the driftcompensated signals are used to light the lamp 55 which illuminates thedisc. When the drift compensator is not in circuit, the stroboscopiclamp I5 is energized directly through a suitable amplifier by means ofswitch H.

Referring to Fig. 2 a detailed description will now be given of atypical picture reproducing equipment such as is indicated in Fig. l bythe numeral I 0, it being assumed for the present that the transmissionis to take place from the picture transmitter over a carrier telephonechannel 6, through the intermediary of the usual telephone carrierdemodulating or detecting equipment 8. Merely for purposes of referenceit will be assumed that the original picture signals, before applicationto the carrier channel, possess a;,fundamental frequency of 1800 cyclesper second, and with amplitude variations corresponding to the shadevaluesof the picture being transmitted. The picture signals afterdemodulation in the device 8 are preferably amplified in a suitableamplifier I8, and part of the amplified signals pass through the filter19' to the amplifier 20, the output of which is impressed upon therecording lamp 2|. Lamp 2| acts on thepicture receiving surface mountedon the drum to reproduce the. picture thereon in successive elementalareas, as disclosed for example in Patent No. 2,015,742, it beingunderstood that the drum 2; is rotated by a suitable synchronous motorll. Another part of the output of amplifier l8 passes through the filter23 and thence to the transformer primary winding 24, the secondarywinding being split as shown and one section of the secondary beingprovided with a condenser so-as to produce a two-phase degree displacedcurrent at the brushes 26, El, 28. These brushes contact withcorresponding Slip rings 29, 3t, 3t connected to the two-phase primaryof the rotary transformer. As shown this primary consists of twowindings 32, 33 mounted for rotation as a unit by a suitable motor. Thesecondary of the rotary transformer comprises the series connectedsingle phase windings 3t, 35 which are mounted for rotation as a unitaround the primary winding, it being understood that suitable rotatingmeans are provided for that purpose. Preferably although notnecessarily, the primary and secondary windings of the rotarytransformer are rotated in the same direction as indicated by thearrows. The current at the brushes 36, Si will therefore be a singlephase alternating current whose frequency is a function of the relativerotational speeds of the primary and secondary windings. By varying thespeed of rotation of either or both of the windings, the frequency ofthe current at brushes 6-36, 3i? may be correspondingly increased ordecreased. If -as assumed above there is no frequency drift, the picturecurrents received over the line t have a frequency of 1800 cycles, andif the windings of the drift compensator are rotated in the samedirection at the same speed, the current at the brushes 35, 3'! willalso be at a frequency of 1800 cycles. This latter current may, ifdesired be amplified in a suitable amplifier 38 and then applied to thesynchronous motor 6 i, and for this purpose the switch til is closed onits upper contact set, and switch Mi on its lower contact set. In orderthat the attendant may ascertain whether the motor ii is operating insynchronism with the signals from the output of amplifier it, said motordrives a strobescopic disc having a series of equally spaced spots 412which are illuminated by the stroboscopic lamp 63. When the switch 40 isclosed on its upper contact set, the lamp 43 which is preferably of theglow discharge type, is excited by the 1800 cycle current supplied fromthe output of amplifiers It and 44. If the spots 42 appear to bestationary then the attendant knows that the motor ii is operating at1800 cycle speed. Under these conditions assuming that there is nofrequency drift introduced at the demodulator 8, and assuming that thewindings of the drift regulator are rotating at the same speed, then theclosure of switch 39 on either its upper or lower contact set should notresult in any apparent movement of the spots 42.

If it is known or suspected that the signals are being transmittedthrough phone circuits involving carrier equipment, the sending operatorwill cause the 1800 cycle carrier to be modulated at a low frequency,say 90 cycles per second. The stroboscope lamp 43 will then illuminatethe target V4! in such a way that the spots 41 will appear prominent. Ifthere is a frequency drift these spots will appear toslowly move with oragainst the rotation of the target. This is an indication to theattendant'that it is necessary to adjust the drift compensator to add orSubtract the necessary number of cycles to cause the target to appear tostand still. Accordingly the attendant causes a relative change in speedbetween the primary 32-'33 and secondary 35. While the above descriptiongives the general method of operation for maintaining synchronism, it ispreferred to employ the following procedure so that before starting theactual transmission and reproduction of a picture the operator at thetransmitter and the operator at the receiver may be assured that theproper synchronous conditions exist. The transmitting operator transmitsan unmodulated 1800 cycle signal which is amplified by amplifier iii.The receiving operator closes switch 39 on its lower contact set, andswitch 40 on its upper contact set and stroboscopically synchronizesmotor H on the 1800 cycle signal by means of the lamp 43 and the disc4|. The transmitting operator then transmits an 1800 cycle signalmodulated with a 90 cycle signal, as represented by the curve of Fig. 3.Under this condition the receiving operator closes switch 45 to connectthe condenser 46 in shunt to lamp 43. The combination of larfip 43 andcondenser 46 acts similar to'a detector of the cycle signal, and asindicated the disc M is provided with a series of larger more widelyspaced spots 41 for stroboscopic synchronizing on 90 cycles. If thespots 41 appear to move-one way or the other the receiving operator isthereby warned that a frequency drift is being introduced in thetransmission channel, whereupon he may adjust the speed of either orboth windings of the drift compensator until the spots again appearstationary. After this adjustment the 90 cycle modulated wave isdisconnected at the transmitter and the system is ready for the regularpicture transmission. If during transmission the receiving operatornotices that the spots 4| appear to move, he may switch in the dritfcompensator by closing switch 39 on its upper contact set and switch 40on its lower contact set. Preferably however, after the initialsynchronization on the 1800 cycle carrier has been effected, the driftcompensator is switched into circuit, so that the actual picturetransmission and reproduction is effected with the drift compensator incircuit at all times.

Under certain conditions it may be desirable instead of employing arelatively high speed synchronous motor such as the 1800 cycle motor Ii,to employ a lower speed for example, 90 cycles. I have found that withsuch lower speeds the frequency drift effect may be avoided bytransmitting the 90 cycles as a modulation of a higher frequency carrierwave, for example a 400 cycle wave. In other words at the transmitterthere will be provided means for producing the usual 1800 cycle picturesignal as described in Patent No. 2,015,742, and in addition there willbe provided a source of 400 cycle carrier current modulated by a 90cycle signal. When the receiver is to be driven from a 90 cycle motorsuch as motor 48 (in place of motor H), switch l!) is closed on itsupper contact set; switch 39 on its lower contact set; and switches 45and B9 are closed. Switch 50 may be closed on either its left-handcontact set or on its right-hand contact set. In the latter position the90 cycle detector 5! is connected in circuit with amplifier 52 and motor58. In certaincases however reliance may be placed entirely on thecombination of condenser 46 and glow lamp M as the detecting mechanism,in which case switch 50 is closed on its left-hand contact set. Ineither case the detected 90 cycle signal will not be affected by anyfrequency drift of the carrier demodulating equipment 8, so that themotor ie may be operated without the drift compensator. It will beunderstood of course that when the motor 63 is being used, that thefilter i9 is properly designed so as to exclude the 400 cycle carrierfrom the picture reproducing lamp 2|, and if desired the filter 23 maybe adjusted to keep the picture frequency of 1800 cycles from reachingthe motor 48.

' Referring to Fig. 4 a description will now be given of a structure ofdrift compensator that has been found to produce the desired results.Two universal variable speed motors 53, 54 with suitable speed governors(not shown) drive respectively the frame 55 and frame 56. Frame 55carries the primary windings 34, 35 (Fig. 4), and frame 56 carries thesecondary windings 32, 33. These frames are rotated in the samedirection but at different speeds depending upon the amount of frequencydrift to be compensated, as described in connection with Fig. 2. Theprimary frame 55.is driven ,through gears 51, 58, and the secondaryframe is driven through gears 59, 60. A thumb screw adjustment 6| ispreferably provided for the governor on motor 54, and this screw may beprovided with a calibrated dial to indicate the speed of motor 54. Ifthe speed of the secondary frameis greater than the primary frame onecycle will be added to the output of the compensator for each revolutiongained by the secondary. If the secondary runs lower than the primary,one cycle willbe subtracted for each revolution lost by the secondary.Instead of using two variable speed motors, one of the motors may be asynchronous motor and the other a variable speed'motor. is preferred toemploy twovariable speed motors of the same matched design sinceordinarily variable speed governor-controlled motors tend to gain orlose in speed after they are warmed up. By using two matched variablespeed motors, the resultant speed differential is reduced to a minimum.and one variable speed motor, two synchronous However, it p Instead ofusing one synchronous motor motors may be employed, one driven from the60 cycle mains and the other driven from a suitable source of variablefrequency, such asan adjustable oscillator.

While certain specific arrangements and types of parts have beendisclosed, it will be understood that various changes and modificationsmay be made therein without departing from the spirit and scope of theinvention by the expression carrier channel as employed in the claims ismeant a communication channel where signals are transmitted by means ofcarrier waves generated at a distant transmitting point and wherein themodulations are demodulated by an oscillator or similar device which isintended to be in synchronism with a similar oscillator at thetransmitting point.

What I claim is: Y

1. In a system for transmitting a visual representation between twopoints employing an oscillator-modulator at the transmitting point andan oscillator-demodulator at the receiving point, the combination of apicture reproducing meansv at the receiving point, a synchronous motordriving said reproducing, means, means to receive synchronizing signalsover the carrier channel, means to indicate frequency drift in saidsynchronizing signals after demodulation at the receiving terminal ofthe carrier channel, frequency changing means to act on the received 2.In a systemfor transmitting a visual representation between stationslinked by a carrier telephone channel of the type having modulating anddemodulating oscillators at the terminals thereof, the combination ofmeans to analyze the representation into corresponding signal currents,means to apply said signal currents to a carrier transmission channel,means to reproduce a picture, under control of said signals, means toindicate frequency drift in said signals and frequency changing meansacting on the signals received from said carrier channel to compensatecontinuously for the frequency drift in troduced into said signals byreason of their transmission over said carrier channel.

3. In a system for transmitting a visual representation between stationslinked by a carrier telephone channel of the type having modulat 'ingand demodulating oscillators at the terminals thereof, the combinationof means to translate a picture into picture current signals having aninherent frequency component related to the speed of the pictureanalyzing mechanism, means to impress said signals on a carriertransmission channel, means to reproduce the picture under control ofsaid signals, the last-mentioned means including movable synthesizingmeans whose speed of operation is controlled by said frequencycomponent, and frequency changing means acting on the signals receivedfrom said carrier channel to compensate for frequency drift introducedinto said frequency component by reason of the transmission over saidcarrier channel.

l. in a system for transmitting a visual repre sentation betweenstations linked by a carrier telephone channel of the type havingmodulatmg and demodulating oscillators at the terminals thereof, thecombination of means to ananism with the analyzing mechanism, the lastmentioned means including a synchronous motor, and a frequency converterwhose output is applied to drive said motor.

5. In a system for transmitting a visual representation between stationslinked by a carrier telephone channel of the type having modulating anddemodulating oscillators at the terminals thereof, the combinationofmeans to analyze a picture into corresponding signal currents havingthe characteristics of a modulated wave of a fre quency corresponding tothe analyzing frequency, a carrier transmission channel having means tomodulate a carrier wave by said signal currents and means to demodulatesaid carrier wave to derive therefrom said currents, a picture reproducing mechanism including synthesizing means and a synchronous motortherefor, a frequency converter on which said derived signals are im-'pressed, and means to operate said synchronous motor by the output ofsaidconverter to compensatefor frequency drift introduced by saidcarrier channel. I

6. In a system for transmitting a picture over a carrier channel of thetype having modulating and demodulating'o'scillators at the terminalsthereof, the method which comprises generating at a transmitter asustained wave modulated by another wave the latter wave having afrequency determined by the speed at which the picture transmitter and'picture reproducer are to operate, applying said modulated wave to acarrier transmission ,channel to transmit said modulated wave as amodulation of a carrier wave over said channel, receiving thefirst-mentioned modulated wave after demodulation from said carrier,stroboscopically synchronizing a device at the receiver under control ofthe modulations in said modulated wave, then transmitting from thetransmitter to the receiver over said channel a sustained wave modulatedin accordance with picture characteristics, applying the last-mentionedwave to synchronize the transmitter and receiver, and acting on saidlast-mentioned wave to compensate for any frequency drift in thelastmentioned wave by reason of its transmission over said carrierchannel.

7. In a system of the character described the combination of means toreceive from a carrier telephone channel of the type having modulatingand demodulating oscillators at the terminals thereof, picture currentsin the form of a sustained wave having amplitude modulationscorresponding tothe shade characteristics of a pic ture to bereproduced,picture reproducing mechanism, a synchronous motor for driving saidmechanism, stroboscopic means to indicate the operation of said motorat'a known frequency, and a frequency converter having its output con--nected to said motor and its input controlled by a portion of saidpicture currents.

8. In a system of the character described, the combination of means toreceive from a carrier telephone channel of the type having modulatingand demodulating oscillators at the terminals thereof, a current havinga frequency for determining the speed at which the picture reproducingmechanism is to operate, picture synthesizing means, means including asynchronous motor to synchronize said synthesizing means at saidfrequency, and frequency converting means Whose output is applied tosaid synchronous motor to control the speed of operation of saidsynthesizing means, and means for applying said current to the input ofsaid converting means.

9. In a system of the character wherein picture currents are transmittedto a receiving station over a carrier telephone channel of the typehaving modulating and demodulating oscillators at the terminals thereof,the combination of a picture scanning mechanism, a synchronous motor foractuating said mechanism, means to receive a synchronizing alternatingcurrent, and .a rotary transformer for converting the frequency of saidcurrent prior to application to said motor to compensate for frequencydrift introduced in said channel, and means to apply the convertedcurrents to said synchronous motor.

10., A system according to claim 9 in which the transformer is providedwith a polyphas'e winding, and means are provided to convert the saidcurrent into a corresponding polyphase current prior to application tosaid transformer.

11. A frequency converter for use in receiving pictures transmitted overa carrier telephone channel of the type having modulating anddemodulating oscillators at the terminals thereof, comprising a,rotatable primary winding and a rotatable secondary Winding, a pairofmatched variable speed motors for driving said windings, means toreceive picture synchronizing signals, meansto apply said signals to theprimary of said converter, means to app the signals in the secondary ofsaid converter to said synchronous 76 motor, and stroboscopic means todetermine whether one or both of said variable speed motors areoperating at the required speed.

12. In a system of the character described, the combination of means totransmit a synchronizing signal over a carrier transmission channel ofthe type having modulating a'nd demodulating oscillators at theterminals thereof, means associated with said channel to demodulate thecarrier to derive the synchronizing signal therefrom, the last-mentionedmeans including a demodulating oscillator, and means to convert thefrequency of said demodulated signal to compensate for the frequencydrift of said oscillator.

13. The method of transmitting a picture over a carrier telephonechannel of the type having modulating and demodulating oscillators atthe terminals thereof which includes the steps of scanning the pictureto produce a synchronizing frequency, modulating said synchronizingfrequency by a lower drift checking frequency, applying said modulatedfrequency to the carrier telephone channel, receiving said modulatedfrequency from said channel, detecting the drift checking frequency andthe synchronizing frequency, driving a scanning device under control ofthe synchronizing frequency, and applying said lower modulatingfrequency to detect stroboscopically whether any frequency drift isintroduced into the synchronizing frequency byisaid carrier telephonechannel.

14. The method according to claim 13 in which the received modulatedsynchronizing frequency is applied to a frequency changer to compensatefor frequency drift introduced by the carrier telephone channel.

15. In a system for transmitting pictures from a transmitting point-to areceiving point over a carrier telephone channel of the type havingcarrier frequency oscillators at the terminals thereof, means at thetransmitting point to generate a voice frequency synchronizing carrier,means to modulate said synchronizing carrier by a lower frequency driftchecking signal, means to apply said modulated carrier to said telephonechannel, means to receive at the receiving point a signal which is theresultant of the transmitted synchronizing frequency and any frequencydrift introduced in the carrier channel, means to detect said lowerfrequency modulation, and stroboscopic means controlled jointly by saidrerultant and said detected modulations.

16. A system according to claim 15 in which a motor is provided fordriving the stroboscope target, said motor being controlled by saidresultant and said detected modulation being applied to a stroboscopelamp for illuminating said target.

17. A system according to claim 15 in which means are provided forapplying the said resultant to an adjustable frequency converter, theoutput of which drives the stroboscope target.

18. A frequency transformer having primary and secondary windingsrotatable in the same direction, each winding having its own drivingmotor and each motor having its own individual speed control.

AUSTIN G. COOLEY.

